The invention relates to a weather radar for measuring radar signals in the GHz range.
Weather radars transmit high-power microwave pulses and measure the strength of the signal that is backscattered by rain or other targets. The rain quantity, for example, can then be estimated from the signal strength. A calibration of the weather radar and, in particular, its receiver is required for this purpose.
From WO 2011/001206 A1 a multichannel receiver system is known allowing for automatic gain calibration by relating the amplitude of a reference signal before coupling it into a receiving path and the amplitude of the reference signal comprised in an output signal of the receiving path and eliminating or reducing changes in conversion gain.
DE 10 2011 012 843 A1 relates to a driver assistance device for a vehicle having a radar unit for detecting a measured variable which determines a frequency-dependent correction quantity for correcting the measured variable.
A polarimetric weather radar system is known from U.S. Pat. No. 7,495,599 B2 which calibrates the signal paths of the two different polarizations by coupling a low-energy test signal at a frequency equal to the radar frequency into both signal paths. The differences in the measured values resulting at the outputs of the two signal paths are then stored as calibration parameters so that the subsequent digital signal processing can take account of and eliminate the difference in the signal paths for real radar signals. The calibration can be carried out either on a one-off basis with a continuous-wave test signal, wherein the antenna signal must be blocked for the calibration period, or on a pulse-by-pulse basis. In this case, a specific range cell is reserved for the calibration, in which a pulsed test signal is then fed in and received instead of the atmospheric echo. A disadvantage of this calibration method is that the data recording of the weather radar must be interrupted during the calibration or range cells must be blocked and excluded from the operational useful signal processing.
From U.S. Pat. No. 8,004,458 B2, a different polarimetric weather radar is known which feeds a test signal at the radar frequency into the signal paths as well. This test signal can be varied in amplitude and phase and thus allows the simulation of various weather conditions. A test signal that is variable to this extent can be used to test and validate the correct function and calibration of the weather radar receiver. For example, Doppler test signals can be fed in or the complex transfer function of the receiver can be determined over the complete dynamic range. This weather radar also has the disadvantage that the data recording must be interrupted during the calibration or range cells must be blocked and excluded from the operational useful signal processing.
From WO 2012/139029 A1, a further polarimetric radar is known which calibrates the two signal paths in ongoing operation without interrupting the data recording. This is done by identifying individual signals in the recorded raw data which correspond to spherical objects. Since spherical objects have an identical reflection behavior for radar waves of both polarizations due to their symmetry, the two signal paths can be calibrated relative to one another on the basis of their reflection signals. This calibration method has the disadvantage that the two signal paths can only be calibrated relative to one another, so that absolute values for calibration parameters, such as amplification or noise, of the individual signal paths and their time dependence cannot be determined. Due to the thermal drifts of the components used in the signal path, measurements at different times are therefore not comparable.